I had this problem this morning. The tech support for our host didn’t have any knowledge of how to fix it but mentioned the when I submitted the form it was generating a fatal PHP error. That gave me the idea that it could be a database error caused by trying to modify the existing form. What also led me to believe this might be the case is when I tried to use a word based quiz instead of the existing number based quiz and was getting the same validation error.

Suspecting it being a database issue trying to modify an existing record I duplicated the form and replaced the quiz shortcode with the recaptcha shortcode and it worked on the first try.

So if you’re trying to enable recaptcha on an existing form and are getting validation errors, try duplicating or recreating the form adding recaptcha.